[MLIR][Parser] Add `parseBase64Bytes`.

This patch adds `parseBase64Bytes` to the parser. It attempts to avoid double-allocating the buffer by re-using the token's spelling directly and eliding the quotes if they exist. It also avoids extra allocations by using std::vector<char> in the API - something we should change when the llvm::decodeBase64 API changes.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D138090